Conventionally, the contact probe used in contact probe assemblies for electrically testing electroconductive patterns of printed circuit boards and electronic components in general comprises an electroconductive needle member, a tubular holder slidably receiving the needle member, and a coil spring for urging the forward end of the needle member out of the tubular holder so that the forward end of the needle member may be brought into contact with an object to be tested with a certain elastic force pushing the needle member against this object.
FIG. 8 shows such a conventional contact probe in which a receptacle 32 serving as a tubular holder is passed through and fixedly secured to an insulator 31 consisting of a plate or block member, and an electroconductive needle member 33 is slidably received in the receptacle 32. The needle member 33 consists of a large diameter portion 33a and a small diameter portion 33b. In FIG. 8, the large diameter portion 33a is provided in a lower part of the needle member 33 while the small diameter portion 33b is provided in an upper part thereof. The upper end of the small diameter portion 33b is provided with an enlarged head 33c, for instance by crimping, and a compression coil spring 34 is coaxially fitted on the small diameter portion 33b between an annular shoulder surface defined by the upper end of the large diameter portion 33a and the head 33c with a certain pre-compression.
The upper end of the head 33c is provided with a coaxial projection 33d, and a lead wire 35 is electrically connected to the needle member 33 via a connector 36 formed as a sleeve member which is securely fitted onto this projection 33d. A central part of the receptacle 32 is provided with an annular projection 37 projecting in a radially inward direction, for instance by crimping, which allows the passage of the head 33c but prevents the passage of the connector 37 and the compression coil spring 34.
Therefore, when the insulator 31 is lowered, and the forward end of the needle member 33 is brought into contact with an object to be tested (not shown in the drawing), the compression coil spring 34 is compressed, and the needle member 33 is pushed against the object to be tested with a suitable pressure. The needle member 33 is prevented from completely coming out of the receptacle 32 by the connector 36 being engaged by the annular projection 37. The electric signal obtained by the needle member 33 is transmitted to a control unit not shown in the drawing via the lead wire 35.
A plurality of such contact probes are normally arranged in parallel in a contact probe assembly so that a plurality of points may be electrically accessed at the same time. In particular, due to the need for increasingly more compact design of electronic equipment, the points to be tested are often extremely densely arranged, and it is desired to minimize the distance between adjacent contact probes in such a contact probe assembly. This can be accomplished by reducing the overall diameter of each contact probe, but it was not possible to reduce the diameter beyond a certain limit imposed by the difficulty in ensuring a sufficient spring force of the compression coil spring, and a sufficient mechanical strength to the needle member.